btrfs: sysfs: add debugging exports

Add 'debug' directories to global sysfs and per-filesystem. This will
replace the debugfs directory. The sysfs location is simpler and builds
on top of the existing file hierarchy so there will hopefully be no more
questions about the sample debugfs file.

The directory is called 'debug' and only present under
CONFIG_BTRFS_DEBUG so this will not affect productions builds.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -248,6 +248,25 @@ static const struct attribute_group btrfs_static_feature_attr_group = {
 	.attrs = btrfs_supported_static_feature_attrs,
 };
 
+#ifdef CONFIG_BTRFS_DEBUG
+
+/*
+ * Runtime debugging exported via sysfs
+ *
+ * /sys/fs/btrfs/debug - applies to module or all filesystems
+ * /sys/fs/btrfs/UUID  - applies only to the given filesystem
+ */
+static struct attribute *btrfs_debug_feature_attrs[] = {
+	NULL
+};
+
+static const struct attribute_group btrfs_debug_feature_attr_group = {
+	.name = "debug",
+	.attrs = btrfs_debug_feature_attrs,
+};
+
+#endif
+
 static ssize_t btrfs_show_u64(u64 *value_ptr, spinlock_t *lock, char *buf)
 {
 	u64 val;
@@ -860,6 +879,13 @@ int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info)
 	if (error)
 		goto failure;
 
+#ifdef CONFIG_BTRFS_DEBUG
+	error = sysfs_create_group(fsid_kobj,
+				   &btrfs_debug_feature_attr_group);
+	if (error)
+		goto failure;
+#endif
+
 	error = addrm_unknown_feature_attrs(fs_info, true);
 	if (error)
 		goto failure;
@@ -952,6 +978,12 @@ int __init btrfs_init_sysfs(void)
 	if (ret)
 		goto out_remove_group;
 
+#ifdef CONFIG_BTRFS_DEBUG
+	ret = sysfs_create_group(&btrfs_kset->kobj, &btrfs_debug_feature_attr_group);
+	if (ret)
+		goto out2;
+#endif
+
 	return 0;
 
 out_remove_group: